A continuously variable transmission comprises a first pulley assembly connected to an internal combustion engine, namely the drive pulley assembly, a second pulley assembly connected to the wheels of the vehicle, namely the driven pulley assembly, and a belt wound between the two pulley assemblies.
Each assembly usually comprises a shaft connected to the motor and wheels, a support element driven by the shaft, and a pair of half-pulleys connected in a rotationally rigid way to the support and mobile in relation to one another along the swiveling support element to define the transmission ratio.
In one possible embodiment, the drive pulley assembly includes a clutch to couple the shaft selectively to the half-pulleys, and the driven pulley assembly is equipped with a cam device to control the relative axial position between the pulleys.
Pulley assemblies provided with cam devices generally include a swiveling sleeve adapted to drive the wheels of the vehicle, a fixed half-pulley connected rigidly to the sleeve, a half-pulley which is axially mobile on the sleeve and driven in rotation by the cam device, and a spring to push the mobile half-pulley towards the fixed half-pulley. In particular, the mobile half-pulley includes a hub supported radially on the sleeve via a bushing, and a conical wall connected coaxially to the hub and adapted to cooperate with the drive belt.
In known pulley assemblies, the cam device comprises a plurality of cams fitted to the sleeve which present a first and second profile converging on the mobile half-pulley, and a cam-follower portion carried by the hub of the mobile pulley which defines a plurality of circumferential cavities that house the corresponding cams with circumferential play. In particular, each cavity is defined laterally by two inclined sides which cooperate with the first or second profile respectively during a drive condition in which the engine drives the wheels, and during an engine-braking condition in which the wheels tend to drive the engine.
During the drive condition, the distance between the half-pulleys of the drive pulley assembly is varied in a way dependent on speed, for example using a centrifugal control device; when the number of revolutions increases, the half-pulleys move closer together, thus increasing the winding diameter. As a result of the action of the belt, the half-pulleys of the driven pulley assembly move apart, compressing the spring, and the mobile half-pulley retracts, performing a partial rotation defined by the cam-follower portion which slides on the first profile of the corresponding cams.
During the engine-braking condition, the sides of the cam-follower portion cooperate with the second profile, which is inclined so as to keep the half-pulleys of the driven pulley assembly close together in order to produce a transmission ratio that maximises the speed of the drive pulley assembly, and thus fully exploit the inertial braking action of the internal combustion engine.
However, in known cam pulley assemblies, problems occur in the transition between the drive condition and the engine-braking condition. In particular, during the initial moments of the engine-braking condition, the cam-follower portion and the mobile half-pulley tend to oscillate circumferentially due to the play between the cams and cavities, and impact alternately on the opposite contact surfaces. Said oscillations generate an undesirable driving feel and noise, which make the comfort of the vehicle worse.